Long-term development of nuclear energetics is associated with production of fast power reactors that can allow solving crucial problems of effective and safe usage of nuclear fuel upon closure of the nuclear fuel cycle and providing environmental safety. Ongoing efforts include the development of new generation lead-cooled fast reactors having uranium-plutonium nitride fuel. The problem of essential design concept selection and provision of such nuclear reactors safety is largely based on the results of researches of various coefficients and reactivity effects which are primarily subjected to nuclear and physical properties of fuel, coolant and other materials, as well as to active zone dimensions and configuration.
In the relevant art there exists a reactor BN-800 with an active zone comprising hexagon-shaped fuel assemblies, wherein the middle part of said fuel assemblies contains uranium-plutonium fuel and the end zones contain upper and bottom breeding blankets (Yu. E. Bagdasarov, L. A. Kochetkov et al. The BN-800 reactor—a new step in fast reactor development. IAEA-SM, No. 284/41, vol. 2, p 209-216, 1985). Inside a vessel of the fuel assembly there are rod-type fuel elements (fuel elements), and within a space between fuel elements in the bottom-to-top direction circulates a coolant, namely molten sodium. A disadvantage of such BN-800 reactor in terms of nuclear safety is a high sodium void reactivity effect. This effect significantly compromises nuclear safety of the reactor in emergencies in the result of which sodium boiling or active zone uncovery occurs.
It is known an active zone of a large fast reactor having a central cavity configured to reduce the sodium void reactivity effect up to its minimum value and ensure the safety of transient processes excluding reactor emergency shut-down (Ru 2126558). The active zone according to this invention comprises fuel assemblies mounted in a circumferential direction and defining a large central cavity; a system of control rods, and devises and materials that can enter inside the cavity in order to emergency shut-down the reactor. The invention enables reducing the void reactivity effect by increasing neutron escape through the large cavity in the central part of the active zone under sodium coolant loss or boiling conditions. However, the use of such active zone can lead to an increase in reactor dimensions and to loss of economic performance.
It is known a modified fast sodium reactor having uranium-plutonium fuel (Ru 2029397). An active zone of this reactor, likewise the active zone of the BN-600 reactor, comprises hexagon-shaped fuel assemblies the middle part of which contains uranium-plutonium fuel and the end zones contain upper and bottom breeding blankets. The central part of each fuel assembly comprises a through-cavity having a diameter which is from 0.3 to 0.8 of the effective diameter of the fuel assembly and extending along an entire height of the active zone and breeding blankets. The rest of the fuel elements are arranged inside a fuel assembly vessel, and in a space between fuel elements in the bottom-to-top direction circulates a coolant, namely molten sodium. In emergencies, such fuel assembly configuration promotes a neutron escape from the reactor active zone into end reflectors, thereby reducing the void reactivity effect. Reactivity reduction and increase of neutron escape via the through-cavity in the fuel assemblies is achieved only by removing a considerable number of fuel elements from the fuel assembly central part. Such solution results in a lower reactor power or in a necessity to enhance nuclear fuel enrichment or increase of active zone dimensions.
It is known a lead-cooled fast reactor which comprises an active zone characterized by zonal distribution of uranium-plutonium nitride nuclear fuel along its radius (Ru 2173484). The nuclear fuel is contained in shells of fuel elements, and a gap between the fuel and the shell is filled with a high thermal conductivity material, for example, lead. The fuel elements are arranged into lead-cooled fuel assemblies. The uranium-to-plutonium mass ratio ranges from 5.7 to 7.3 and is uniform across the entire active zone. The fuel in the active zone is radially zoned, and the active zone comprises at least two subzones: a central and a peripheral. The peripheral subzone has more fuel and less coolant than the central subzone. Distribution of the nuclear fuel and the coolant between the subzones is performed by means of changing a pitch between the fuel elements and/or by using in the central and peripheral parts fuel elements of different diameters. The upper parts of the fuel elements comprise gaseous cavities with a height of minimum 0.8 of a fuel column height.
The invention enables to obtain uniform fuel burnup and plutonium breeding rates at the central and peripheral parts of the active zone, lower the temperature difference between the fuel elements and the coolant along the radius and increase the nuclear safety of the reactor in a case of emergency, for example, coolant loss. The configuration of the reactor, the active zone, fuel assemblies and fuel elements described in detail in the invention contemplates further technical solutions, in particular those allowing to lower a reactor reactivity margin to an optimal level, improve heat transfer from the fuel to the fuel element shells, lower thermomechanical interaction of the fuel with the fuel element shells, reduce pressure inside the fuel elements. Zonal distribution of uranium-plutonium nitride fuel and coolant along the active zone radius according to the invention disclosed in the patent Ru 2173484 is provided either by using in the central or peripheral fuel assemblies the fuel elements of different diameters and/or by using different packing density thereof. Thus, in particular, the ratio between diameters of the fuel elements in the fuel assemblies of the peripheral subzones and the fuel elements in the fuel assemblies of the central subzones is equal to 1.12, and the pitch ratio between the fuel elements in the fuel assemblies of the central subzone and between the fuel elements in the fuel assemblies of the peripheral subzone is equal to 1.18. In such way, the practical application of the present invention is driven by a need of nuclear fuel production and use of fuel elements and fuel assemblies of different dimensions resulting in increase of costs for nuclear fuel production.